"Open in Browser" was added to the context menu for opening a website from RSS (bug 688745)

Attachments on compose windows can be renamed using F2 on Windows and Linux now (bug 718310)

Addresses can be dragged from the the address book sidebar now (bug 713405)

Pasting a URL from the clipboard into the Download Manager window will download it (bug 719196)

The Feed Subscribe dialog remembers where you place it now (bug 712035)

A Print button can be added to the compose window toolbar now (bug 718517)

Spelling settings now have their own preferences pane under Appearance (bug 527173)

Plugins can be disabled for the whole suite now in addition to Mail & Newsgroups only (bug 620776)

Support for other message storage formats (pluggable mail stores) has been added (bug 402392).To enable the new Maildir format (instead of the default Mbox) for new accounts, set mail.serverDefaultStoreContractID to "@mozilla.org/msgstore/maildirstore;1". There is no UI for any of this yet.